1. Field of the Invention
This invention relates to polyurethane dispersions, and more particularly, to coagulated polyurethane dispersions useful as coating compositions.
2. Description of the Prior Art
Natural leather, appropriately finished, is valued for its durability and aesthetic characteristics in a plurality of uses. Due to the scarcity of leather and the increased cost of processing leather for particular applications, economics have dictated that synthetic materials be substituted in certain applications where leather goods had been used. Such synthetic materials have been proposed and used in the areas of shoe uppers, upholstery, clothing, luggage making, book binding and similar applications. Because these various applications require varying physical, chemical and aesthetic qualities, different processes using different materials must be used to obtain an acceptable product which is comparable to natural leather; although in most instances these synthetics are readily distinguishable from natural leather.
One method of preparing a synthetic as a substitute for leather involves impregnating and/or coating a porous material, for example, cloth, with a polyurthane, vinyl or a similar material. Polyurethanes have met with wide acceptance as a coating or impregnating composition due to their capability of wide variation in chemical and physical properties, particularly their flexibility and chemical resistance.
Objectives in preparing these synthetic substitutes for leather are that they provide: (1) sheets especially suitable for leather-like and upholstery uses; (2) sheets of uniform width as commonly used in the textile industry (unlike natural products which sustain substantial weight and area losses in cutting and finishing); (3) end use versatility, for example, under a variety of exposure conditions where certain chemical treatments will assist maintenance and useful lifetime of properties; (4) and most importantly, a product with the strength, hand, drape and softness comparable to natural leather.
Further, an impregnated fabric sheet material when used for shoe uppers, should be characterized by a leather-like appearance, with no undesirable fabric show-through, good water vapor permeation into the uncoated inside of the upper, and the leather-like grain break (minimal gross wrinkling). "Leather-like grain break", as recognized in the leather and upholstery industries, is manifested in the behavior of well finished leather when folded or crumpled. The leather fold is characterized by a smooth, curved contour, frequently with numerous fine wrinkles in the compressed region of the fold area. This is contrasted with sharp creases or gross wrinkles formed when papers or films are folded; this kind of undesirable appearance is known as "pin wrinkling".
In another application it is desired to provide a strengthened fabric that can be sanded or buffed, giving an aesthetically pleasing surface and then used without additional coating as an upholstery material. In upholstery, important characteristics include strength, reduced bias elongation, and enhanced appearance, i.e. a well filled fabric, without substrate show-through, which has softness, drape and conformability. The bias elongation is important in shoe uppers as well as upholstery, and reflects a desirable resistance to stretch in the more highly stressed areas of the fabric in the end use; excessive stretch in a fabric will result in pin wrinkling and show-through.
Polyurethane polymers as coatings or impregnants for fabric have long been recognized as providing some of the aforementioned characteristics. For example, polyurethanes can be made which are highly resistant to solvents and abrasion, conferring dry-cleanability and outstanding durability to coated fabrics. The basic chemistry of polyurethanes, involving reactions between the isocyanate groups and molecules with multiple reactive hydrogens, such as polyols and polyamines, afford great versatility and variability in final chemical and physical properties by the selection of intermediates to achieve processability and the desired balance of end use performance requirements.
There are various methods for applying polyurethane solutions or other post curable liquid polymers to porous substrates which are well known to those skilled in the art. An article in Journal of Coated Fabrics, Vol. 7 (July 1977), pp. 43-57 describes some of the commercial coating systems, e.g. reverse roll coating, pan fed coater, gravure and the like. Brushing and spraying may also be used to coat polyurethanes on porous substrates. These polyurethane solutions, after impregnation or coating on the porous substrate, are dried or cured by a method such as heated air, infrared radiation and the like. Characteristic of these processes is the deposition of a polymer and a film-like layer which tends to produce a coated fabric which folds in undesirable sharp creases rather than a leather-like grain break.
Another method of combining polyurethane solutions with porous substrates is disclosed in U.S. Pat. No. 3,208,875. Briefly, this method involves the application of a solution of a polymer in an organic solvent to a substrate (such as a needle punched polyester batt) with subsequent bathing of the polymeric layer with a mixture of an organic solvent for the polymer and a non-solvent for the polymer that is at least partially miscible with the solvent until the layer is coagulated into a cellular structure of interconnected micropores. The solvent is removed from the coating layer along with the non-solvent to produce a solvent-free microporous layer. Although this process yields acceptable properties for a polyurethane impregnated fabric, it has the disadvantages of an organic solvent system, particularly when high performance polyurethanes are utilized which require relatively toxic and high boiling solvents.
Polyurethane dispersions in organic vehicles have been proposed and used to coat fabrics. U.S. Pat. No. 3,100,721 discloses dispersions made by the addition of non-solvent to polyurethane solutions. A dispersion, applied to a substrate, is coagulated by further addition of a non-solvent.
In another aspect of forming coated fabrics with aesthetically pleasing end properties along with adequate physical properties, fabrics have been coated with a foam or microporous layer and then subsequently finished with a pigmented material along with a clear finish coat. This approach eliminates the "telegraph" of the fibers of the substrate and enables a regular surface to be imparted to the fabric which was initially an irregular surface. Direct coating of fabrics and other irregular surfaces results in the telegraphing effect in which the irregular surface of the initial fabric shows through the coated product. Exemplary of the approaches of forming a microporous layer on the fabric is shown in U.S. Pat. No. 3,632,417. Further approaches are exemplified by U.S. Pat. No.3,418,198 which teach that a microporous interlayer on the fabric be provided having a smoothness factor of not more than 15 mils. This smoothness factor of 15 mils means that 15 mils or less of the dried coating that is coated upon the substrate will provide a smooth coating which hides the pattern or texture of the coated fabric.
In still another prior art method, such as disclosed in U.S. Pat. No. 3,418,198, a "non-roughening" impregnated material is proposed.
Various methods of impregnating fabrics and non-woven sheet materials have been utilized. Among these methods are those disclosed in U.S. Pat. No. 4,171,391 entitled "Method of Preparing Composite Sheet Material"; U.S. patent application Ser. No. 188,329, filed Sept. 18, 1980 by John McCartney, along with several modifications thereof as is diclosed in U.S. Pat. No. 4,332,710. These approaches have achieved a certain degree of success in forming leather-like materials; however, it is desirable in some instances to coat some of these substrates with a microporous layer in order to form a regular surface which does not telegraph any of the imperfections or textile pattern of the fibrous substrate.
In accordance with the present invention, a coating composition is provided which forms a microporous layer on an irregular surface or substrate, thus eliminating this telegraphing or roughening effect.
Further in accordance with the present invention, a coating composition for irregular surfaces is provided which can be directly finished, thereby eliminating the need for a further coating of the microporous layer.
These and other advantages are achieved by the invention.